If sale deed is executed on a mortgage deed which not registered,is it valid sale deed.

My father was cheated by the ccop bank,he was not the sole owner of the flat.he took a loan thru an agent.for which he had mortgaged his flat for 10 lakh rs for term loan.its mentioned in the mortgage deed.which is not registered but stamp duty is paid. sir 40 lakh was cash credit facility for which stok n good in shop were mortgaged tangible moveable property agreement stamps r there but not registered.my father has paid the money rs.10 lacs for term loan a/c n had told the bank to return the mortgage deed but bank credited this amt rs.10 lakhs in his cash credit a/c n didnt return him the mortgage deed.he kept requesting but due to his medical condition couldnt do the follow up. my sister n bro in law knew all the facts and they were suppose to file cases against the fraud bank who had also taken the stock n goods that were mortgaged for the cash credit facility.bank gave n.o.c to sell the flat to my father.my sister has made an register sale deed n has mentioned that she has executing her sale deed on a REGISTERED mortgage deed as my father is willing to sell the flat n clear the loans.she has paid the loan even when she knew his father had cleared the term loan n had bank dudnt return him that mortgage deed.bank didnt do any physical verification from society office before disbursement. sir my sister has not taken the releae deeds of other 8 shareholders of the flat.its my grandmothers flat.my sister has now filed an eviction suit against us.my sister has not taken any n.o.c to sell,n.oc to transfer or n.o.c to mortgage the flat from the housung sciety .society has given us their on our letters reply thru their adv that my fathers transfer was provisional n for sake of convinniance n my sisters transfer is also for sake of convininace.my grand mother had died intestated.there r no titles that prove that my father was the sole owner if the flat.other share holders objection r lying in the society office since 1980.sister had always hidden her sale deed made between herself my father n banks mortgage deed.my another sister has filed injunction suit in city civil.so my sisrer who us also a defandant had to submitt this sale deed at evidence stage for ownership.sir ur delay was condoned after 10 years in that suit as v had medical condition sir is my sister n bro in laws registered sale deed valid or its a fraud as she has executed on a UNREGISTERED mortgage deed n even after knowing the facts she has not taken release deed of other 8 share holders.will she get the ownershop of the flat.